1. Field of the Invention
This invention relates to the joining of pipes by means of explosive welding, and particularly to a method of explosive welding in which a disintegrating anvil is used.
2. Summary of the Prior Art
It has been known for many years to join lengths of pipe by means of explosive welding. This technique has been found to be particularly useful in joining large and long pipes such as are used for commercial pipe lines. Three basic welding systems are used, these being:
(i) systems in which two coaxial tubes constrained on the outside by a die are welded when an explosives charge is detonated in the bore of the inner cylinder;
(ii) systems in which two coaxial cylinders, supported from the inside by a rigid plug, are welded when an explosive charge is detonated on the outside surface of the outer tube; and
(iii) systems in which two coaxial cylinders are welded when two explosive charges, placed respectively inside and outside the cylinder assembly, are detonated.
As indicated above, the second system requires the use of a rigid plug which acts as a contact anvil or mandrel inside the inner pipe to provide a reaction to the force generated by the explosion to prevent collapse of the inner pipe during welding. This anvil can be in the form of a solid plug which is subsequently removed, or it may be in tubular form and remain inside the tube as a permanent mandrel. The solid plugs were typically made of a low melting point metal, such as lead, which could be melted out after the welding was completed. However, melted lead is not an easy material to handle, particularly when extremely long lengths of pipe are being joined. The permanent mandrel has the disadvantage of producing a restriction in the inside of the pipe. This is tolerable for gas transmission and distribution lines but is not tolerable for pipelines which carry a variety of products, both liquid and gas, because the permanent mandrel would interfere with the "pigging" operation used to clean the pipelines.
There has, therefore, been a need in the field for an improved form of removable anvil or mandrel. 15 An attempt at producing such an improved form of anvil is described in U.S. Pat. No. 4,228,941, issued Oct. 21, 1980. In that case, the anvil was made from a salt which is soluble in water. This salt anvil could be removed by dissolving in water after the explosive welding was completed. Although the U.S. patent describes what is basically a good idea, the only example given of a suitable salt for the purpose was a salt mixture of sodium nitrite, sodium nitrate and potassium nitrate. The inventor suggested that the salt anvil can be enclosed in a protective cover which is not permeable, or can be provided with a protective layer which is not permeable. He further suggested using a plastic for the cover and a latex or varnish for the protective layer. He particularly specified that the cover or layer must not be soluble in water.
The present inventor has found that the salt mixture used in the U.S. patent is highly hygroscopic and must be protected from moisture during storage and use. The most serious disadvantage of the insoluble protective coating or cover, necessary because of the hygroscopicity, is that the anvil is no longer completely soluble, leaving portions of the insoluble coating within the pipeline after the anvil is dissolved following explosive welding.
It is, therefore, the object of the present invention to provide a salt capable of producing an anvil which has the necessary strength characteristics to be used in explosive welding and being fully soluble in water, while avoiding the disadvantage of being highly hygroscopic.